Project 1 Summary One of the key challenges confronting the accurate diagnosis and effective treatment of cancer is its heterogeneous nature. This is particularly true with bladder cancer (BC) which is now known to comprise principal histological types (transitional cell carcinoma, squamous cell carcinoma, adenocarcinoma and small cell carcinoma), major phenotypic variants within the most prevalent transitional cell carcinoma (low-grade papillary BC and high-grade invasive BC) and molecular subtypes within the muscle-invasive BC (such as luminal-subtype and basal-subtype). Importantly, different BC entities appear to be quite divergent in biological behavior, clinical outcome and response to therapies. However, the exact mechanism(s) underlying BC heterogeneity remain poorly understood, thus presenting a major hindrance for rapid and meaningful clinical translation of the basic science discoveries. The present proposal is therefore designed to address this important and pressing problem by specifically targeting a set of genetic and molecular events highly prevalent in human BC into different cell populations of murine urothelia. Specific Aim 1 will interrogate key mutational events that activate the RTK-PI3K-RAS pathway, in conjunction with the loss of tumor suppressors in the 9p21 locus, in the context of the formation of low-grade papillary BC. Aim 2 will dissect the combinatorial driver effects of p53 deficiency along with altered DNA repair genes/histone modifiers, in the context of the formation of high-grade invasive BC and potentially its subtypes. The experimental approaches will combine the generation and in-depth characterization of transgenic, knockin, knockout and compound mice. Together, the proposed studies should significantly enhance our understanding of the genetic, molecular and cellular bases of BC heterogeneity. This should in turn lead to the development of new biomarker panels that can more reliably stratify BC variants, more accurately predict their probability of progression and likelihood to respond to chemo-, radio- and immuno-therapeutics. The true genetic, molecular and cellular identification of BC variants should also help uncover new targets for therapeutic intervention.